Portal Image Analysis Research Articles

SU‐D‐BRA‐05: Time Series Analysis of EPID Images to Identify Patients in Need of Treatment Adaptation

Purpose:to evaluate if time series analysis of portal dose images can be used to identify patients undergoing important anatomical changes.Methods:daily EPID images of every treatment fields were acquired for 48 patients treated for lung cancer. In addition, CBCT were acquired on a regular basis (weekly or biweekly). Gamma analysis was performed relative to the first fraction given that no significant anatomical change was observed on the CBCT of the first fraction compared to the planning CT. Several parameters were extracted from the gamma analysis (e.g. average gamma value, standard deviation, percent above 1). The gamma parameters formed a patient‐specific time series that was analyzed. The first 24 patients were retrospectively evaluated to establish an action threshold that was then applied on the remaining 24 patients. Dosimetric evaluation of patients above that threshold was performed to as assess the level of degradation compared to the initial treatment plan.Results:after performing a clinical retrospective analysis of the first 24 cases, an action threshold on the average gamma value was established at 0.6 and a warning level was set at 0.4. These thresholds were then applied to the remaining 24 cases. Of these, 6 patients (25%) were above the warning level and 4 (17%) were above the action threshold. Dosimetric evaluation was performed on the CBCT for all these 6 patients. Three of these patients had changes above 3% in their PTV coverage, one had changes of about 2% and the remaining two had negligible changes. Patients with the strongest changes all had clear trending in their gamma parameters that could be classified with techniques such as hidden Markov models.Conclusion:by using time series analysis of relative EPID image it was possible to identify a subset of patient most likely to benefit from treatment adaptation.This work was funded in part by Varian Medical Systems

Tissue motion tracking at the edges of a radiation treatment field using local optical flow analysis

This paper investigates the feasibility and accuracy of tracking the motion of an intruding organ-at-risk (OAR) at the edges of a treatment field using a local optical flow analysis of electronic portal images. An intruding OAR was simulated by modifying the portal images obtained by irradiating a programmable phantom's lung tumour. A rectangular treatment aperture was assumed and the edges of the beam's eye view (BEV) were partitioned into clusters/grids according to the width of the multi-leaf collimators (MLC). The optical flow velocities were calculated and the motion accuracy in these clusters was analysed. A velocity error of 0.4 ± 1.4 mm/s with a linearity of 1.04 for tracking an object intruding at 10mm/s (max) was obtained.

TH‐A‐BRA‐07: Optimization of Gamma Parameters for EPID‐Based Pretreatment Detection of Delivery Errors in IMRT Fields

Purpose: Gamma analysis is widely used for intensity modulated radiation therapy (IMRT) quality assurance. However, when calculated on a discrete grid with conventional dose/distance criteria of f_dose = 3%/f_dist = 3mm, gamma exhibits poor correlation with dose errors and poor specificity/sensitivity for detecting fluence deviations. This work shows how gamma criteria may be optimized for error detection, and uses repeat images of 11 clinical IMRT fields to quantify delivery errors that can be reliably detected via optimal analysis of electronic portal (EPID) images. Methods: Multiple EPID images were acquired of 11 clinical IMRT fields. For each field, images were aligned and averaged, generating a reference image. Gamma was computed for individual images relative to reference using two methods: discrete grid (0.37mm̂2 pixel size) and continuous (interpolation‐ free). Rectangular errors were inserted into measured images, and receiver operating characteristic (ROC) studies determined parameters that yield maximum image classification accuracy. Classification was deemed reliable if =95% of images were correctly classified. Results: Key parameters are f_grad=0.37xf_dose/f_dist and the gamma distribution tail cutoffs used to classify images as errored or error‐free. f_grad controls the ‘looseness' of the distance‐to‐agreement (DTA) search. Classification is optimal when f_grad is loose enough to ignore normal image‐to‐image variations, but tight enough to detect delivery errors. Conventional 3%/3mm criteria give sub‐ optimal error detection. Even with optimal parameters, calculation on a discrete grid introduces sufficient sampling errors as to seriously degrade error detection. If continuous gamma calculation is employed with optimal parameters, one can reliably detect small errors, e.g., dose deviations of 5‐ 10% over areas of 3×3 pixels (∼1mm̂2) at isocenter. Conclusions: Optimal detection of IMRT delivery errors using EPID images is achieved with continuous gamma, and optimal criteria (f_dose, f_dist and classification cutoffs). Caution must be exercised when gamma is calculated on a discrete grid using conventional 3%/3mm criteria. Research funding from Varian Medical Systems.

SU‐GG‐J‐09: Effective Tracking of Intrafractional Organ Motion Due to Breathing Using a Siemens 160 MLC

Purpose: In this work we demonstrate effective tracking of highly irregular breathing motion using a dynamic Siemens 160 MLC™ and a neural network predictor to compensate for the relatively large end‐to‐end system latency of 500 ms. Method and Materials: One‐dimensional breathing patterns were recorded from several radiotherapy patients using the ANZAI pressure belt system. The pressure belt signal was reproduced by a programmable phantom, which has embedded electromagnetic transponders for target localization and tracking (Calypso® Medical Technologies, Inc.). The aperture of a Siemens 160 MLC™ was adapted in real‐time to the target position provided by the Calypso system. Due to the highly irregular nature of breathing motion, advanced prediction methods are required to accurately compensate the system latency. We therefore implemented a feedforward neural network predictor. The network was trained on a 120 second sample of breathing data which preceded the data that was used for the subsequent tracking experiments. The geometric tracking accuracy was determined through an analysis of portal images acquired during radiation delivery. A metal ball attached to the phantom allowed the reconstruction of the phantom trajectory from the portal images. Tracking errors were quantified as the difference between the phantom trajectory and the geometric centroid of the radiation field on the portal images. Results: In an offline cross‐validation study, we could identify a common set of model parameters of the neural network, which yielded good prediction performance for all the breathing patterns under consideration. The analysis of the images acquired during the tracking experiments yielded root mean squared tracking errors of 1.21 mm, 1.29 mm and 1.58 mm for the three investigated motion patterns. Conclusion: In spite of the relatively large system latency, we achieved highly accurate MLC tracking for irregular breathing patterns.Conflict of Interest: Supported by Siemens Healthcare OCS and Calypso Medical Technologies, Inc.

Potentials of on-line repositioning based on implanted fiducial markers and electronic portal imaging in prostate cancer radiotherapy

BackgroundTo evaluate the benefit of an on-line correction protocol based on implanted markers and weekly portal imaging in external beam radiotherapy of prostate cancer. To compare the use of bony anatomy versus implanted markers for calculation of setup-error plus/minus prostate movement. To estimate the error reduction (and the corresponding margin reduction) by reducing the total error to 3 mm once a week, three times per week or every treatment day.Methods23 patients had three to five, 2.5 mm Ø spherical gold markers transrectally inserted into the prostate before radiotherapy. Verification and correction of treatment position by analysis of orthogonal portal images was performed on a weekly basis. We registered with respect to the bony contours (setup error) and to the marker position (prostate motion) and determined the total error. The systematic and random errors are specified. Positioning correction was applied with a threshold of 5 mm displacement.ResultsThe systematic error (1 standard deviation [SD]) in left-right (LR), superior-inferior (SI) and anterior-posterior (AP) direction contributes for the setup 1.6 mm, 2.1 mm and 2.4 mm and for prostate motion 1.1 mm, 1.9 mm and 2.3 mm. The random error (1 SD) in LR, SI and AP direction amounts for the setup 2.3 mm, 2.7 mm and 2.7 mm and for motion 1.4 mm, 2.3 mm and 2.7 mm. The resulting total error suggests margins of 7.0 mm (LR), 9.5 mm (SI) and 9.5 mm (AP) between clinical target volume (CTV) and planning target volume (PTV). After correction once a week the margins were lowered to 6.7, 8.2 and 8.7 mm and furthermore down to 4.9, 5.1 and 4.8 mm after correcting every treatment day.ConclusionProstate movement relative to adjacent bony anatomy is significant and contributes substantially to the target position variability. Performing on-line setup correction using implanted radioopaque markers and megavoltage radiography results in reduced treatment margins depending on the online imaging protocol (once a week or more frequently).

An analysis of geometric uncertainty calculations for prostate radiotherapy in clinical practice

Margins are used in radiotherapy to assist in the calculation of planning target volumes. These margins can be determined by analysing the geometric uncertainties inherent to the radiotherapy planning and delivery process. An important part of this process is the study of electronic portal images collected throughout the course of treatment. Set-up uncertainties were determined for prostate radiotherapy treatments at our previous site and the new purpose-built centre, with margins determined using a number of different methods. In addition, the potential effect of reducing the action level from 5 mm to 3 mm for changing a patient set-up, based on off-line bony anatomy-based portal image analysis, was studied. Margins generated using different methodologies were comparable. It was found that set-up errors were reduced following relocation to the new centre. Although a significant increase in the number of corrections to a patient's set-up was predicted if the action level was reduced from 5 mm to 3 mm, minimal reduction in patient set-up uncertainties would be seen as a consequence. Prescriptive geometric uncertainty analysis not only supports calculation and justification of the margins used clinically to generate planning target volumes, but may also best be used to monitor trends in clinical practice or audit changes introduced by new equipment, technology or practice. Simulations on existing data showed that a 3 mm rather than a 5 mm action level during off-line, bony anatomy-based portal imaging would have had a minimal benefit for the patients studied in this work.

Radiation therapists’ perceptions of the minimum level of experience required to perform portal image analysis

Background and purpose Our aim was to explore radiation therapists’ views on the level of experience necessary to undertake portal image analysis and clinical decision making. Materials and methods A questionnaire was developed to determine the availability of portal imaging equipment in Australia and New Zealand. We analysed radiation therapists’ responses to a specific question regarding their opinion on the minimum level of experience required for health professionals to analyse portal images. We used grounded theory and a constant comparative method of data analysis to derive the main themes. Results Forty-six radiation oncology facilities were represented in our survey, with 40 questionnaires being returned (87%). Thirty-seven radiation therapists answered our free-text question. Radiation therapists indicated three main themes which they felt were important in determining the minimum level of experience: ‘gaining on-the-job experience’, ‘receiving training’ and ‘working as a team’. Conclusions Radiation therapists indicated that competence in portal image review occurs via various learning mechanisms. Further research is warranted to determine perspectives of other health professionals, such as radiation oncologists, on portal image review becoming part of radiation therapists’ extended role. Suitable training programs and steps for implementation should be developed to facilitate this endeavour.

First clinical experience with cone-beam CT guided radiation therapy; evaluation of dose and geometric accuracy

The uncertainty in the position of mobile organs during radiotherapy limits the accuracy of treatment. Therefore several groups developed image guidance methods for correction of the target position using markers, ultrasound, and in-room CT. Recently, we integrated a cone-beam CT on a linear accelerator (Elekta Synergy Research Platform) in our hospital. The purpose of this paper is to present our first clinical experience, evaluating dose, image quality, and geometric accuracy in 28 patients. For a complete CT reconstruction, data are acquired over 30–250 s (80–700 projections), depending on the required image quality. The CT reconstruction (using in-house developed image guidance software) of a 256 × 256 × 256 volume (with 1 mm cubic voxel size) takes about 20 s. With a central detector position the field of view is 25 cm × 25 cm × 25 cm. Offsetting the detector by 11 cm increases the field of view to 40 cm × 40 cm × 25 cm. At the increased field of view a 360 degree gantry rotation is required, while for the smaller field of view about 190 degree gantry rotation suffices for a proper CT reconstruction. The dose was quantified using an ionization chamber inserted in a Rando-phantom. For quality assurance of the geometric accuracy and image quality studies, 140 cone-beam scans were made of 28 patients, mostly treated for prostate, head and neck, and lung cancer. The setup error was determined by automatically registering bony anatomy in the cone-beam CT scan to the planning CT scan, taking planning data (isocenter) into account. For all patients, setup errors determined with the cone beam system were compared with those determined using electronic portal images (of the same fraction) and DRRs. Automatic soft tissue registration for prostate has been implemented. Prostate calcifications were present in 3 patients, and were used as inherent markers to validate the prostate localization accuracy. The dose required to scan a head and neck patient with good image quality was 1 cGy (central) and 1.8 cGy skin dose. Doses for a pelvic patient were slightly lower. The image quality was sufficient to visualize soft tissues such as lung tumors, rectum, and bladder. The prostate was visible, but not in all slices. Scan truncation artifacts (because the patient shadow does not fit on the detector) caused Hounsfield unit calibration problems but hardly influenced image quality or image registration. The cone beam data clearly revealed setup error, as well as anatomical deformations due to neck flex and tumor regression. Comparison between cone beam data and portal image derived setup errors showed differences of about 1 mm SD for all three axis for head and neck and prostate. These differences were mainly attributed to observer variation in portal image analysis. In addition there was a systematic error of 1 mm in the cranio-caudal direction, caused by inherent inaccuracy of the MV isocenter. For lung cancer patients, the differences were around 3 mm SD, indicating a much larger observer variation in portal image analysis for this patient group. To evaluate patient motion and respiration differences between cone beam CT and portal images, DRRs made of cone-beam data were directly compared with the portal images. These differences were smaller than 1 mm. The accuracy of automatic prostate tracking using cone beam data was 1.7 mm SD (AP direction) and better in other directions. Our clinical validation showed that imaging dose and geometric accuracy compare favorable with electronic portal imaging. For this reason, electronic portal imaging has been replaced by cone-beam imaging on this accelerator since February 2004. At present, an off-line positioning protocol for bony anatomy setup is in clinical use. The next step is to implement soft-tissue based setup corrections clinically. Protocols for prostate (based on automatic prostate tracking) and lung (based on respiration correlated cone-beam CT) are in development

Do confidence, formal training or years of experience influence the accuracy of electronic portal image review by radiation therapists?

background: verification of radiation therapy treatment using electronic portal images (epi) involves the correlation between a reference image and a treatment epi. this paper investigates the accuracy and confidence in reference anatomy outlining and anatomy matching components of epi review, when performed by radiation therapists with varying experience in epi analysis.methods: thirty radiation therapists performed reference anatomy contouring and anatomy matching of seven pairs of reference and treatment images belonging to five previous patients. accuracy was determined by the discrepancy (mm) between the anatomy contouring and matching of the subjects and the original parameters completed by an experienced senior radiation therapist, with formal training in epi analysis. confidence was recorded on a 10cm visual-analogue scale (vas).results: no significant differences were found in the accuracy of anatomy contouring or anatomy matching between subjects with and without formal epi training nor between subjects with less than or greater than five or more years of radiation therapy experience (p > 0.18). significantly different confidence scores were found in subjects with formal training in epi (p < 0.00006)conclusions: accuracy of anatomy contouring and matching was not significantly influenced by the years of radiation therapy experience and formal epi training, although these factors did affect subjects' confidence in performing epi review tasks. frequent exposure to portal image analysis may be as or more important to the technical skills required in epi review, than formal training or years of experience.

Improvements in radiotherapy practice: the impact of new imaging technologies.

Improvements in imaging technology are impacting on every stage of the radiotherapy treatment process. Fundamental to this is the move towards computed tomography (CT) simulation as the basis of all radiotherapy planning. Whilst for many treatments, the definition of three-dimensional (3D) tumour volumes is necessary, for geometrically simple treatments virtual simulation may be more speedily performed by utilising the reconstruction of data in multiple imaging planes. These multi-planar reconstructions may be used to define both the treatment volumes (e.g. for palliative lung treatments) and the organs at risk to be avoided (e.g. for para-aortic strip irradiation). For complex treatments such as conformal radiotherapy (CFRT) and intensity-modulated radiotherapy (IMRT) where 3D volumes are defined, improvements in imaging technologies have specific roles to play in defining the gross tumour volume (GTV) and the planning target volume (PTV). Image registration technologies allow the incorporation of functional imaging, such as positron emission tomography and functional magnetic resonance imaging, into the definition of the GTV to result in a biological target volume. Crucial to the successful irradiation of these volumes is the definition of appropriate PTV margins. Again improvements in imaging are revolutionising this process by reducing the necessary margin (active breathing control, treatment gating) and by incorporating patient motion into the planning process (slow CT scans, CT/fluoroscopy units). CFRT and IMRT are leading to far closer conformance of the treated volume to the defined tumour volume. To ensure that this is reliably achieved on a daily basis, new imaging technologies are being incorporated into the verification process. Portal imaging has been transformed by the introduction of electronic portal imaging devices and a move is underway from two-dimensional (2D) to 3D treatment verification (cone beam CT, optical video systems). A parallel development is underway from off-line analysis of portal images to the incorporation of imaging at the time of treatment using image-guided radiotherapy. By impacting on the whole process of radiotherapy (tumour definition, simulation, treatment verification), these new imaging technologies offer improvements in radiotherapy delivery with the potential for greater cure rates and a minimum level of treatment side effects.

Intrafractional stability of the prostate using a stereotactic radiotherapy technique

Purpose To evaluate the stability of the prostate during stereotactic radiation therapy. Methods and materials Forty-seven patients underwent placement of three fiducial markers into the prostate as part of a pilot study of hypofractionated stereotactic radiotherapy. Portal images before and subsequent to 227 radiotherapy fractions were analyzed for prostate movement. Six patients also underwent localizing radiographs at 6-min intervals for 24 min. Relative motion of the bony landmarks and prostate markers was calculated. Results Analysis of portal images revealed the undirected average prostate movement of 2.0 mm (superior/inferior), 1.9 mm (anterior/posterior), and 1.4 mm (right/left) with maximum standard deviation (SD) of 2.0. Analysis of radiographs at 6-min intervals showed the greatest undirected average prostate motion between 0–6 min; 1.5 mm (superior/inferior), 1.4 mm (anterior/posterior), and 0.4 mm (right/left). Beyond 6 min, movements decreased to 0.4, 0.9, and 0.8 mm, respectively. Bony landmark motion was 0.9 mm (superior/inferior), 0.9 mm (anterior/posterior), and 0.4 mm (right/left) between 0–6 min. Beyond 6 min, motion decreased to less than 0.5 mm in any direction. Conclusions Stereotactic prostate radiotherapy, utilizing fiducial marker localization, resulted in average intrafractional prostate movement of 2.0 mm or less. Most patient and organ movement occurs early and a settling-in period is advisable before treatment.

250 poster Determination of transfer errors from treatment planning CT to linear accelerator by 3-D portal image analysis of phantom measurements

250 poster Determination of transfer errors from treatment planning CT to linear accelerator by 3-D portal image analysis of phantom measurements

A simple and non-invasive vacuum mouthpiece-based head fixation system for high precision radiotherapy.

To demonstrate why conventional non-invasive mouthpiece-based fixation has not achieved the expected accuracy and to suggest a solution of the problem. The Vogele Bale Hohner (VBH) head holder is a non-invasive vacuum mouthpiece-based head fixation system. Feasibility and repositioning accuracy were evaluated by portal image analysis in 12 patients with cranial tumors intended for stereotactic procedures, fixated with the newest version (VBH HeadFix-ARC). Portal image analysis (8 patients evaluated in 2-D, 4 patients in 3-D) showed that even in routine external beam radiation therapy, treatment can be applied to within a mean 2-D and 3-D accuracy of under 2 mm (SD 0.92 mm and 1.2 mm, respectively) with cost and repositioning time per patient and patient comfort comparable to that of common thermoplastic masks. These preliminary results show that high repositioning accuracy does not rule out simple and quick application and patient comfort. Paramount, however, is tensionless repositioning via the vacuum mouthpiece.

Application of anisotropic diffusion to digital enhancement of portalimages

We propose the use of anisotropic diffusion filtering to remedy difficultiesin analysis of electronic portal images, stemming from their low contrast andhigh noise levels. Anisotropic diffusion is a nonlinear filter based on thenumerical solution to the partial differential equation describing the processof diffusion. In this study we show that this filter is capable of greatlyreducing noise in homogeneous areas of portal images while preserving theedges and contrast associated with anatomical features. We also demonstratethat the application of anisotropic diffusion leads to more consistent andreproducible visual extraction of features from portal images.

3-D portal image analysis in clinical practice: an evaluation of 2-D and 3-D analysis techniques as applied to 30 prostate cancer patients

Purpose: To investigate the clinical importance and feasibility of a 3-D portal image analysis method in comparison with a standard 2-D portal image analysis method for pelvic irradiation techniques. Methods and Materials: In this study, images of 30 patients who were treated for prostate cancer were used. A total of 837 imaged fields were analyzed by a single technologist, using automatic 2-D and 3-D techniques independently. Standard deviations (SDs) of the random, systematic, and overall variations, and the overall mean were calculated for the resulting data sets (2-D and 3-D), in the three principal directions (left–right [L-R], cranial–caudal [C-C], anterior–posterior [A-P]). The 3-D analysis included rotations as well. For the translational differences between the three data sets, the overall SD and overall mean were computed. The influence of out-of-plane rotations on the 2-D registration accuracy was determined by analyzing the difference between the 2-D and 3-D translation data as function of rotations. To assess the reliability of the 2-D and 3-D methods, the number of times the automatic match was manually adjusted was counted. Finally, an estimate of the workload was made. Results: The SDs of the random and systematic components of the rotations around the three orthogonal axes were 1.1 (L-R), 0.6 (C-C), 0.5 (A-P) and 0.9 (L-R), 0.6 (C-C), 0.8 (A-P) degrees, respectively. The overall mean rotation around the L-R axis was 0.7°, which deviated significantly from zero. Translational setup errors were comparable for 2-D and 3-D analysis (ranging from 1.4 to 2.2 mm SD and from 1.5 to 2.5 mm SD, respectively). The variation of the difference between the 2-D and 3-D translation data increased from 1.1 mm (SD) for zero rotations to 2.7 mm (SD) for out-of-plane rotations of 3°, due to a reduced 2-D registration accuracy for large rotations. The number of times the analysis was not considered acceptable and was manually adjusted was 44% for the 2-D analysis, and 6% for the 3-D analysis. Conclusion: True 3-D analysis of setup errors for a group of 30 patients with prostate cancer demonstrated that setup rotations are rather small. The deformation of the projected anatomy in portal images caused by out-of-plane rotations leads to a reduced 2-D registration accuracy. For rotations larger than 3° this effect can be quite pronounced, making 3-D registration the preferred method. Furthermore, the automatic 3-D registration has a higher success rate, most likely because this technique uses more information compared to the 2-D method.

Automatic on-line electronic portal image analysis with a wavelet-based edge detector.

A fully automatic method for on-line electronic portal image analysis is proposed. The method uses multiscale edge detection with wavelets for both the field outline and the anatomical structures. An algorithm to extract and combine the information from different scales has been developed. The edges from the portal image are aligned with the edges from the reference image using chamfer matching. The reference is the first portal image of each treatment. The matching is applied first to the field and subsequently to the anatomy. The setup deviations are quantified as the displacement of the anatomical structures relative to the radiation beam boundaries. The performance of the algorithm was investigated for portal images with different contrast and noise level. The automatic analysis was used first to detect simulated displacements. Then the automatic procedure was tested on anterior-posterior and lateral portal images of a pelvic phantom. In both sets of tests the differences between the measured and the actual shifts were used to quantify the performance. Finally we applied the automatic procedure to clinical images of pelvic and lung regions. The output of the procedure was compared with the results of a manual match performed by a trained operator. The errors for the phantom tests were small: average standard deviation of 0.39 mm and 0.26 degrees and absolute mean error of 0.31 mm and 0.2 degrees were obtained. In the clinical cases average standard deviations of 1.32 mm and 0.6 degrees were found. The average absolute mean errors were 1.09 mm and 0.39 degrees. Failures were registered in 2% of the phantom tests and in 3% of the clinical cases. The algorithm execution is approximately 5 s on a 168 MHz Sun Ultra 2 workstation. The automatic analysis tool is considered to be a very useful tool for on-line setup corrections.

Initial clinical experience with a video-based patient positioning system

Purpose: To report initial clinical experience with an interactive, video-based patient positioning system that is inexpensive, quick, accurate, and easy to use. Methods and Materials: System hardware includes two black-and-white CCD cameras, zoom lenses, and a PC equipped with a frame grabber. Custom software is used to acquire and archive video images, as well as to display real-time subtraction images revealing patient misalignment in multiple views. Two studies are described. In the first study, video is used to document the daily setup histories of 5 head and neck patients. Time-lapse cine loops are generated for each patient and used to diagnose and correct common setup errors. In the second study, 6 twice-daily (BID) head and neck patients are positioned according to the following protocol: at am setups conventional treatment room lasers are used; at pm setups lasers are used initially and then video is used for 1–2 minutes to fine-tune the patient position. Lateral video images and lateral verification films are registered off-line to compare the distribution of setup errors per patient, with and without video assistance. Results: In the first study, video images were used to determine the accuracy of our conventional head and neck setup technique, i.e., alignment of lightcast marks and surface anatomy to treatment room lasers and the light field. For this initial cohort of patients, errors ranged from σ = 5 to 7 mm and were patient-specific. Time-lapse cine loops of the images revealed sources of the error, and as a result, our localization techniques and immobilization device were modified to improve setup accuracy. After the improvements, conventional setup errors were reduced to σ = 3 to 5 mm. In the second study, when a stereo pair of live subtraction images were introduced to perform daily “on-line” setup correction, errors were reduced to σ = 1 to 3 mm. Results depended on patient health and cooperation and the length of time spent fine-tuning the position. Conclusion: An interactive, video-based patient positioning system was shown to reduce setup errors to within 1 to 3 mm in head and neck patients, without a significant increase in overall treatment time or labor-intensive procedures. Unlike retrospective portal image analysis, use of two live-video images provides the therapists with immediate feedback and allows for true 3-D positioning and correction of out-of-plane rotation before radiation is delivered. With significant improvement in head and neck alignment and the elimination of setup errors greater than 3 to 5 mm, margins associated with treatment volumes potentially can be reduced, thereby decreasing normal tissue irradiation.

Comparison of computer workstation with light box for detecting setup errors from portal images

Purpose: Observer studies were conducted to test the hypothesis that radiation oncologists using a computer workstation for portal image analysis can detect setup errors at least as accurately as when following standard clinical practice of inspecting portal films on a light box.Methods and Materials: In a controlled observer study, nine radiation oncologists used a computer workstation, called PortFolio, to detect setup errors in 40 realistic digitally reconstructed portal radiograph (DRPR) images. PortFolio is a prototype workstation for radiation oncologists to display and inspect digital portal images for setup errors. PortFolio includes tools for image enhancement; alignment of crosshairs, field edges, and anatomic structures on reference and acquired images; measurement of distances and angles; and viewing registered images superimposed on one another. The test DRPRs contained known in-plane translation or rotation errors in the placement of the fields over target regions in the pelvis and head. Test images used in the study were also printed on film for observers to view on a light box and interpret using standard clinical practice. The mean accuracy for error detection for each approach was measured and the results were compared using repeated measures analysis of variance (ANOVA) with the Geisser–Greenhouse test statistic.Results: The results indicate that radiation oncologists participating in this study could detect and quantify in-plane rotation and translation errors more accurately with PortFolio compared to standard clinical practice.Conclusions: Based on the results of this limited study, it is reasonable to conclude that workstations similar to PortFolio can be used efficaciously in clinical practice.

Portfolio: a prototype workstation for development and evaluation of tools for analysis and management of digital portal images

Purpose: The purpose of this investigation was to design and implement a prototype physician workstation, called PortFolio, as a platform for developing and evaluating, by means of controlled observer studies, user interfaces and interactive tools for analyzing and managing digital portal images. The first observer study was designed to measure physician acceptance of workstation technology, as an alternative to a view box, for inspection and analysis of portal images for detection of treatment setup errors.Methods and Materials: The observer study was conducted in a controlled experimental setting to evaluate physician acceptance of the prototype workstation technology exemplified by PortFolio. PortFolio incorporates a windows user interface, a compact kit of carefully selected image analysis tools, and an object-oriented data base infrastructure. The kit evaluated in the observer study included tools for contrast enhancement, registration, and multimodal image visualization. Acceptance was measured in the context of performing portal image analysis in a structured protocol designed to simulate clinical practice. The acceptability and usage patterns were measured from semistructured questionnaires and logs of user interactions.Results: Radiation oncologists, the subjects for this study, perceived the tools in PortFolio to be acceptable clinical aids. Concerns were expressed regarding user efficiency, particularly with respect to the image registration tools.Conclusions: The results of our observer study indicate that workstation technology is acceptable to radiation oncologists as an alternative to a view box for clinical detection of setup errors from digital portal images. Improvements in implementation, including more tools and a greater degree of automation in the image analysis tasks, are needed to make PortFolio more clinically practical.

High-precision prostate cancer irradiation by clinical application of an offline patient setup verification procedure, using portal imaging

Purpose : To investigate in three institutions, The Netherlands Cancer Institute (Antoni van Leeuwenhoek Huis [AvL]), Dr. Daniel den Hoed Cancer Center (DDHC), and Dr, Bernard Verbeeten Institute (BVI). how much the patient setup accuracy for irradiation of prostate cancer can be improved by an offline setup verification and correction procedure, using portal imaging. Methods and Materials : The verification procedure consisted of two stages. During the first stage, setup deviation were measured during a number ( max) of consecutive initial treatment sessions. The length of the average three dimensional (3D) setup deviation vector was compared with an action level for corrections, which shrunk with the number of setup measurements. After a correction was applied, N max measurements had to be performed again. Each institution chose different values for the initial action level (6, 9, and 10 mm) and N max (2 and 4). The choice of these parameters was based on a simlation of the procedure, using as input preestimated values of random and systematic deviations in each institution. During the second stage of the procedure, with weekly setup measurements, the AvL used a different criterion (“outlier detection”) for corrective actions than the DDHC and the BVI (“sliding average”). After each correction the first stage of the procedure was restarted. The procedure was tested for 151 patients (62 in AvL, 47 in DDHC, and 42 in BVI) treated for prostate carcinoma. Treatment techniques and portal image acquisition and analysis were different in each institution. Results : The actual distribution of random and systemic deviations without corrections were estimated by eliminating the effect of the corrections. The percentage of mean (systematic) 3D deviations larger than 5mm was 26% for the AvL and the DDHC, and 36% for the BVI. The setup accuracy after application of the procedure was considerably improved (percentage of mean 3D deviations larger than 5 mm was 1.6% in the AvL and 0% in the DDHC and BVI), in agreement with the results of the simulation. The number of corrections (about 0.7 on the average per patient) was not larger than predicted. Conclusion : The verification procedure appeared to be feasible in the three institutions and enable a significance reduction of mean 3D setup deviations. The computer simulation of the procedure proved to be a useful tool, because it enable an accurate prediction of the setup accuracy and the required number of corrections.